By labeling the soluble matrix (the stroma) of tobacco cell plastids with green fluorescent protein, it was possible in previous work to visualize thin tubular projections which emanate from the plastid surface and sometimes interconnect with other plastids, as well as to demonstrate that interconnected plastids can exchange protein through these plastid tubules (K[unreadable]hler et al. 1997). FCS provides a means to characterize the transport properties of such interplastid protein exchange. The effect of inhibitors like Cyanide, Colchicin and Cytochalesin D on the transport parameters is tested and quantified.